In the pharmaceutical industry, it is important to know the parameters which characterize the tableting properties of powders which are compressed to form tablets used for human and animal consumption. Of these parameters, the bonding index of the powder is probably the most important. This bonding index is determined from fundamental considerations of specific tablet models in order to determine how the inter-particle bonds form. With respect to the experimental tests which are carried out on these models, these typically involve an indentation hardness test wherein a spherical indenter is impacted against one face of the model, and a tensile strength determination wherein the model is subjected to transverse compression.
The hardness impact testers typically are of a pendulum type and comprise a ball or spherical indenter of known mass and diameter suspended from a wire, a tablet holding structure for maintaining the tablet in a fixed position during the test, and a device for determining the rebound height of the ball or spherical indenter after striking the tablet. The mass of the spherical indenter, the radius of the spherical indenter, the chordal radius of the dent produced in the tablet by impact with the spherical indenter, the initial height of the indenter and the rebound height of the indenter is then inserted into an equation in order to give the indentation hardness of the tablet.
In Powder Technology, 38 (1984) 145-159, Hiestand and Smith, there is disclosed an impact hardness tester which comprises a steel sphere suspended from a one-meter long steel wire, a die containing the compressed tablet mounted directly below the pendulum suspension point, and a polaroid camera for determining the rebound height of the spherical indenter after impact with the tablet. This apparatus also contains an electromagnet for holding and releasing the sphere and a shutter which when activated, prevents a second impact of the sphere against the tablet.
With this early model impact hardness tester, it is difficult to accurately determine the rebound height of the sphere from the time-lapse photographs. Also the tablet is subject to fracture upon impact by the sphere, which fracture can dissipate some of the energy of the sphere and, therefore, decrease the rebound height and yield incorrect values in the calculations of the dynamic indentation hardness and the strain index.
A further pendulum-type hardness impact tester is described in an article by R. 0. Williams III, and J. W. McGinity. This latter impact hardness tester modified the basic apparatus described by Hiestand and Smith by substituting a ballistic sensor for the polaroid camera to measure the velocity of the sphere before and after impact with the tablet. This latter tester also uses a split die assembly to minimize triaxial decompression of the tablet during the hardness test. Although these modifications do improve the operation of the impact hardness tester, the basic problems with the apparatus are still unresolved.
More specifically, the ballistic sensor measures the velocity of the spherical indenter by timing the interruption caused by the equator of the spherical indenter passing between a solid state infrared light source and a detector. The time of this interruption and the diameter of the sphere are then used to compute the velocity of the sphere. This necessitates the adjustment of the ballistic sensor each time a spherical indenter of a different diameter is used, and also introduces an additional calculation to be made in the determination of the hardness of the tablet. Furthermore, although the split die arrangement for holding the tablet helps alleviate the fracture problems, nevertheless undesirable fractures still occur.
Accordingly, it is an object of this invention to provide an improved pendulum-type impact hardness tester for measuring the hardness of tablets or compacts from a powdered material, which impact hardness tester improves upon the performance characteristics of the known impact hardness testers described above.
More specifically, the improved hardness impact tester has an improved split die assembly which has an upper die portion connected to a pressure cylinder which enables the regulation of the die wall pressure. The improved impact hardness tester also contains an improved rebound height detector which comprises photocells for determining the rebound velocity of the spherical indenter after impact with the tablet.
Other objects and purposes of the invention will be apparent to persons familiar with impact hardness testers of this general type upon reading the following specification and inspecting the accompanying drawings.